The present invention relates generally to a gas turbine system for generating high-temperature process heat. More specifically, the present invention relates to a gas turbine system for coal gasification, in which at least one primary circuit having a heat source, a heat exchanger and a recycle blower, is connected via the heat exchanger to a secondary circuit having at least one process-gas compressor, at least one process-heat exchanger and an expansion turbine. The system also includes a drive circuit with a gas turbine, at least one cooler and a compressor.
For the endothermic conversion of coal into synthetic gas, nuclear energy can be used, in an advantageous way, as a supply source for process heat. However, a gas, preferably helium, serving as the reactor coolant of a high-temperature reactor cannot be used directly, although a temperature level of approximately 950.degree. C. is viewed as an aim in the development of reactors of this type. To meet the requirements for safety and for isolation of the process-heat part of such a system, especially with regard to radioactive contamination, a heat exchanger would have to be provided. Heat exchangers which meet the high safety requirements demanded cannot be made for such high temperatures, in the present state of the art, primarily because of metallurgical problems.
Also, the helium which flows out of the process-heat part of the system and which still has a temperature of approximately 750.degree. C. could not serve as the reactor coolant. To achieve a realistic temperature for the individual structural parts and for the control and shut-down rods, the helium would therefore have to be cooled further, to 300.degree.-400.degree. C.
A gas turbine system of the type mentioned above is known from German Offenlegungsschrift No. 2,826,315. In the circuit shown in FIG. 6 of the Offenlegungsschrift, the cooling-gas temperature at the outflow from the reactor is so low that the gas can be conveyed without previous expansion into the heat exchanger of the primary circuit. Since mechanical energy is required to drive the circulating pump in the primary circuit and the process-gas compressor in the secondary circuit, a special drive ciruit in the form of a gas turbine system is integrated within the primary circuit. This gas turbine system has a turbine, a compressor, a generator and a cooler and is dimensioned so that its output covers the energy requirement for the turbo-machines of the primary and secondary circuits.
Such a system, however, has some serious disadvantages. In the first place, all the machines and apparatus of the drive circuit mentioned are located in the nuclear circuit, thus making it difficult to house and maintain them. Furthermore, all the useful energy has to be generated electrically and then reconverted mechanically, which involves losses and also necessitates large electrical machines. Moreover, mixing losses occur when the drive circuit is brought into the primary circuit. Finally, thermodynamic losses are unavoidable because the expansion turbine in the secondary circuit can have only a low pressure ratio owing to the high inflow temperature of the process gas on the secondary side on entry into the heat exchanger.
One primary object of the present invention is, therefore, to simplify the primary circuit and thus to avoid the necessity of having the same circulation means for the primary circuit and for the drive circuit.
In a gas turbine system of the type mentioned in the introduction, this is achieved, according to the present invention, by a combination of the following features:
(a) immediately downstream of the heat exchanger, the medium heated in the secondary part of the latter is divided into a portion of drive gas for operating the drive circuit and a portion of process gas for the actual generation of process heat in the secondary circuit;
(b) after the process heat generated in the process-gas compressor has been given off in the process-heat consumer part and then expanded in the expansion turbine, the process gas is introduced into the drive circuit at a junction having at least approximately the same thermodynamic state;
(c) the combined drive gas and process gas, compressed in the compressor of the drive circuit, is pre-heated in a recuperator before entry into the secondary part of the heat exchanger;
(d) the expansion turbine in the secondary circuit drives, in addition to the process-gas compressor, a generator which generates the electrical energy required to drive the recycle blower.
One advantage of the present invention is to be seen in the fact that, in addition to the free choice of the heat source, not only gas cooling, but also metal cooling, for example by means of liquid unpressurized sodium, can be employed for cooling the heat source. Furthermore, many degrees of freedom are available for the introduction of the expanded process gas into the drive circuit.
It is also advantageous if the expanded process gas gives off part of its residual heat in an aftercooler or in the recuperator before it is combined with the drive gas.
According to a further embodiment of the gas turbine system according to the present invention, the mass flow of the drive gas is calculated so that the gas turbine subjected to it supplies only the power required to drive the compressor generating the circuit pressure.